The present invention relates to a valve drive of an internal combustion engine that operates according to the “rotating actuator principle.”
Such a valve drive is known from German Patent DE 101 40 461 A1. With traditional internal combustion engines, the camshaft is driven mechanically by the crankshaft via a control chain or a control belt. To increase the engine power and to reduce fuel consumption, a considerable advantage would be achieved by individually triggering the valves of the individual cylinders or at least the intake valves and outlet valves of the individual cylinders. This is possible by means of an electromagnetic valve drive. In an electromagnetic valve drive, an “actuator unit” is assigned to each valve and/or each “valve group” of a cylinder. Different basic types of actuator units are currently being researched. In one basic type, an opening magnet and a closing magnet are assigned to one valve or one valve group. The valves can be displaced axially, i.e., opened and/or closed, by applying electric current to the magnets. However, such valve drives are difficult to control from the standpoint of control technology. With the other basic type, a control shaft with a cam is provided, whereby the control shaft can be pivoted back and forth by an electric motor. This is also referred to as the so-called “rotating actuator principle.” In German Patent DE 101 40 461 A1 cited above, the cam acts on a rocker arm. Then the opening force generated by the cam is applied by the rocker arm to the valve. At the end of the control shaft, a lever-like element is also provided in the form of a “hand crank.” Furthermore, a spring clip is also provided, having a protruding spring arm that presses against the lever-like element. The spring arm of the pivoting lever exerts a torque on the control shaft and/or on the cam. The torque depends on the position of the lever-like element, i.e., the pivot position of the control shaft.
As mentioned above, the control shaft together with the cam in the case of a valve drive like that described in German Patent DE 101 40 461 A1 pivots cyclically back and forth. A reversal of direction of rotation is thus occurring constantly. The electric motor here must accelerate the control shaft and the cam and the lever-like element attached thereto out of the resting state to a relatively high rotational speed. Although the electric motor is supported by the spring clip in opening the valve, it must work against the force of the locking spring, which requires a relatively high electric power. One essential problem here is that the electric motor “starts” from the resting state each time in acceleration of the control shaft, the cam and the lever-like element of the electric motor connected to the control shaft. It takes a certain amount of time with each cycle until the electric motor reaches a rotational speed at which the electric motor operates with a favorable electric efficiency. At low rotational speeds in particular, the efficiency of the electric motor is relatively unfavorable, resulting in a high power consumption.
The object of the present invention is to create an electric valve drive that operates according to the “rotating actuator principle” which is improved with regard to the electric power consumption.
The starting point for the present invention is a valve drive for an internal combustion engine with a valve that is arranged so that it is axially displaceable between an open position and a closed position. Due to a locking spring, the valve is prestressed in the direction of its closed position. Furthermore, a control shaft is provided with a cam which operates the valve. The control shaft is coupled to an electric motor that pivots the control shaft back and forth about a longitudinal axis. Furthermore, a pivotably mounted “pressure element” prestressed by a spring is provided. The pressure element prestressed by the spring exerts a torque on the control shaft. The torque exerted instantaneously on the control shaft depends on the pivot position of the cam. In the back and forth movement of the control shaft, the pressure element is also pivoted back and forth about its pivot axis.
The present invention is based on the finding that the power required for operation of the valve and/or the electric power required for valve operation depends to a significant extent on the ratio of the moments of mass inertia of the “pivotable valve drive components.” The greater the mass moment of inertia of the control shaft and of the cam, the more power must be supplied by the electric motor for acceleration of the control shaft and the cam. In opening the valve, the acceleration of the control shaft and the cam is supported by the pressure element prestressed by the spring. When the valve is closed, then the spring is maximally stressed. It can be demonstrated in an experiment that the mass moment of inertia of the pressure element in particular and/or the mass moment of inertia formed by the spring and the pressure element have a decisive effect on the electric power required for operation of the electric motor. A good “electric efficiency” is achieved when the mass moment of inertia of the pressure element in relation to its pivot axis is greater than the mass moment of inertia formed by the control shaft and the cam in relation to the longitudinal axis of the control shaft.
Thus, the pressure element is designed to be “more solid” than would actually be necessary for transmission of the prestressing force generated by the spring.
In designing the electric motor, it is advantageous not to set the maximum rotational speed of the electric motor too high. In fact, the maximum rotational speed of the electric motor could be reduced with an increase in the mass moment of inertia of the control shaft and the cam. As already mentioned, however, the dynamics of the valve drive decreases with an increase in the mass moment of inertia of the control shaft and the cam because the mass moment of inertia of the control shaft and the cam must first be accelerated electrically by the electric motor and then must additionally be accelerated mechanically by the springs because the mass moment of inertia of the control shaft and of the cam must also be accelerated even in the “stable end positions,” i.e., from the resting positions of the control shaft. Likewise, this mass moment of inertia must be accelerated electrically in the case of a “mini stroke operation.”
However, an increase in the mass moment of inertia of the pressure element has the advantage that the pressure element need not be accelerated out of the resting position by the electric motor alone when opening the valve but instead is also moved by the spring element.
During an initial phase of the opening procedure of the valve, first the control shaft and the cams are accelerated by the electric motor to a certain speed without the valve already being opened. During this initial phase, the pressure element is also accelerated and thus stores a certain amount of rotational energy. The actual opening movement of the valve, when the valve is opened against the locking spring force of the valve, begins during the second phase. The energy required for opening the valve is applied primarily by the spring element and the “kinetic energy” stored in the pressure element.
By increasing the mass moment of inertia of the pressure element, more kinetic energy is stored in the pressure element accordingly. This part of the energy need no longer be stored in the camshaft. In other words, according to this invention, a portion of the energy required for opening the valve is “shifted” from the camshaft to the pressure element. This permits a reduction in the maximum control shaft rotational speed required for valve opening. The increase in the mass moment of inertia of the pressure element acts like an increase in the mass moment of inertia of the control shaft in this operating state. Since the “rotational actuator” must be accelerated electrically out of the two end positions, a low mass moment of inertia is favorable with regard to actuator dynamics as well as with regard to electric power consumption, in particular at the start of the acceleration movement.
Another advantage achieved with the present invention lies in the fact that the average rotational speed of the electric motor with this invention is shifted into a higher rotational speed range. Therefore, the ohmic losses are reduced, especially in acceleration of the electric motor from low rotational speeds, thus resulting in an improvement in overall electric efficiency. The total power consumption declines and the amount of lost heat to be dissipated is thus also reduced.
According to one embodiment of the present invention, the spring element is a torsion spring. This may be a torsion spring rod whose first end is fixedly clamped, e.g., being attached to an actuator housing with the pressure element attached to its other end and protruding essentially perpendicularly away from the torsion spring rod. The torsion spring rod may be arranged in parallel with respect to the control cam which is thus a very space-saving arrangement.
The “elevated” mass moment of inertia of the pressure element is preferably achieved by a mass concentration at the end distal from the torsion spring. This yields a relatively high mass moment of inertia with a comparatively low total mass of the pressure element. The pressure element may be manufactured from a plate-shaped component, for example, and may have a closed contour with a recess in the central area. The pressure element may be a punched part. The recess may be punched out of the central area in particular.
As already mentioned, according to the present invention, the mass moment of inertia of the pressure element in relation to its pivot axis is greater than the mass moment of inertia formed by the control shaft and the cam and in relation to the longitudinal axis of the control shaft. An especially favorable mass moment of inertia ratio is obtained when the mass moment of inertia of the pressure element in relation to its pivot axis is greater by a factor in the range between 1.7 and 2.3 than the mass moment of inertia formed by the control shaft and the cam and in relation to the longitudinal axis of the control shaft.
Other objects will become apparent for one skilled in the art on seeing the description and claims.